When the oil of an electrical transformer becomes contaminated or properties of the oil deteriorate, there is a need to filter the oil. Since the oil is always filtered from the bottom of the electrical transformer and fed again into to the conservator or top filter valve of the electrical transformer tank, oil circulation is blocked whenever filtered oil is fed into the conservator, whose outlet conduit or pipe is fitted with a conventional design preset non return valve. Therefore filtration of oil cannot be done. In the existing or conventional design of preset non return valve (PNRV), isolation of the electrical transformer conservator occurs in case of bursting of electrical transformer tank or rupture of electrical transformer bushing mounted on the electrical transformer tank. However, during filtration or filling or refilling of electrical transformer oil, the PNRV by virtue of its inherent construction remains closed and prevents flow of oil. It is therefore necessary to obviate the above drawback of the prior art system by another device.